G1

Basic Plasma Research – Proposals for Applications
プラズマの基礎的研究−応用への提案
Noriyoshi Sato
佐藤徳芳
Professor Emeritus, Tohoku University
Home: Kadan 4-17-113, Sendai 980-0815, Japan
東北大学名誉教授
自宅：〒980-0815 仙台市青葉区花壇 4-17-113
Some basic experiments are presented for next-stage intelligent plasma applications. They include works
on productions of large-scaled discharge plasmas for uniform plasma processing, controls of electron
temperature for finding processing conditions, and removal of dust particles for clean processing.

1. Introduction
The author has been interested in basic plasma
research to establish “intelligent” plasma processing
for device and material manufacturing.
Here some basic experiments [1] are introducedon plasma productions for large-scaled uniform
plasma processing under different pressure ranges,
electron-temperature controls for finding conditions
required in plasma processing, and dust-particle
removal necessary for a new clean technology.
Several drastic results obtained are presented toshow that the methods proposed could be useful inmany kinds of plasma processing in the future.
2. Plasma Productions
2-1. Under Pressure < 0.1 TorrIn the experiments here, non-uniform plasmas areproduced for large-scaled uniform plasma processing at a distance not far from the position of plasma
production. A part of the vacuum chamber is used
as an electrode or an antenna. Microwave and rf
plasmas have been produced on this line of research.
The density profiles are controlled by permanent
magnets. They provide also magnetic fields for
ECR in case of the microwave plasmas and formodified magnetron typed (MMT) discharge incase of the rf plasmas. A feedback control could beFig. 1. MMT reactor developed at Hitachi Kokusai Electric Inc.

effective for meter-size uniform processing.
The MMT reactor developed for semiconductor
manufacturing [2] is demonstrated in Fig. 2.
2-2. Under Pressure of 0.1 ~ 10 Torr
A parallel-plate rf discharge under this pressure
range has been widely used for plasma productionsin applications. Multi-hollows formed in the rf
powered electrode is effective for increasing theplasma density. But, the discharge is often localized
in the special hollow(s). There is also a possibilityof dust-particle trapping in the isolated hollows.
To eliminate these problems in the isolatedhollow cathode, the hollows are connected by ditch[3]. Gas-feed holes are made in the bottoms of thehollows and/or between the hollows in the hollow
cathodes. The plasma density has been confirmed toincrease with no localization of the discharge incase of the connected-hollow cathode.
2-3. Under Atmospheric Pressure
Plasma processing using atmospheric plasmas is
now quite useful for various applications. So-called“barrier discharges” are well known as a method ofplasma production under the atmospheric pressure.
We have proposed a quite simple method of theatmospheric plasma production [4]. This is called
CCMD (Capacity-Coupled Multi-Discharges, seeFig. 2). Needle-typed electrodes are set near a metalplate, which are coupled with external capacitors.
Being different from the barrier discharges, thedischarge power can be controlled to increase byincreasing the capacity of the capacitors.

Fig. 2. Circuit for CCMD.

3. Electron-Temperature Control
We have proposed two methods for electrontemperature (Te) control. They were established byvarying local plasma structures to yield high andlow Te regions.
One of them employs a pin-hollow cathode. A
radial discharge structure is varied by the length ofpins installed axially near the inside wall of thehollow cathode. With an increase in the length, theionization localizes gradually near the pins, being
followed by the Te decrease in the central region.
A grid is employed in the other method, which
separates the discharge region (I) from a region (II)for plasma processing. In the presence of a negative
potential applied to the grid, most electrons in theregion I are reflected by the grid except high-energy
tail electrons. But, there occurs ionization due to thetail electrons in the region II. Electrons produced
there are not responsible for maintaining thedischarge, having a low Te. Therefore, Te in theregion II decreases with an increase in the negative
grid potential. Te is controlled also by varying thegrid mesh size or the size of hole(s) made in thegrid even at a floating grid potential.
Both of the methods yield continuous Te controls
over one to two orders of magnitude. Drastic effectsof Te on reactive plasmas have been observed in theexperiments. A lot of negative hydrogen ions (morethan 90%) are produced in the low Te region inhydrogen and methane plasmas, suggesting a
possibility of efficient negative ion source.
Figure 3 presents an effect of Te on formation of
diamond particles in hydrogen-methane plasmas.
High-quality diamonds are formed with a decreasein Te even at low gas pressure around 0.1 Torr.
The Te-control is also useful for producing ahigh-quality a-Si:H film for solar-cell battery [5].

removal of fine particles in dusty plasmas. TheCollector is just a simple electrode with hole(s),which is biased higher than the floating potential inplasmas. Particles (<50µm) pass through the hole(s)without impinging the electrode surface into thehole(s). When particles near the Collector arecollected, particles left away approach the Collector
in the presence of force balance among particles,
being pulled one after another into the Collector.
Fine particles levitating in the horizontal plane
above a metal plate show a spatial distribution,
depending on the potential profile above the platesurface. We can prepare the surface with ditches
shaped to yield a potential profile for particles to be
guided toward the NFP-Collector. Then, theCollector can be located at a position far away fromthe central plasma region, as shown by its example
in Fig. 4. Even if the ditches are filled up withinsulator or the plate is covered with an insulator
film to yield the plane surface, we can have almost
the same results for the particle removal.
Since the NFP-Collector removes fine particles
before they grow large, the Collector is also useful
to suppress the particle growth in reactive plasmas.

Fig. 4. Ditches for guiding particles toward the NFP-Collector.

4. Conclusions
Basic plasma experiments have been extended topropose new methods for plasma applications. They
are expected to be useful in the future.
Acknowledgments
The author appreciates the collaborations at
Tohoku University, ANELVA Corporation, Hitachi
Kokusai Electric Inc., SHARP Corporation, andADTEC Plasma Technology Co., Ltd.

Fig. 3. Effect of electron temperature on diamond formation.

4. Dust Removal
As an extension of experiments on negative-ionplasmas and fullerene plasmas, the author wasengaged in various basic works on fine-particle
plasmas. On the basis of the physics clarified, theNFP-Collector (Negatively-Charged Fine-ParticleCollector) has been proposed for collection andReferences[1] N. Sato, Proc. of 28th Annual Meeting of Instituteof Electrostatics Jpn., 2004, p. 35 [in Japanese];
Dust Plasmas in Application (Odessa, 2004), p. 11.
[2] U. Ogawa, N. Sato, K. Shino, and R. Furukawa,Hitachi Hyoron 85 (2003) 41 [in Japanese].
[3] T. Koshimizu, U. Ogawa, O. Kasahara, and N.
Sato, Proc. of 63th Annual Meeting of Jpn. Soc.
Appl. Phys., No.1, 2002, p. 87 [in Japanese].
[4] H. Mase, T. Fujiwara, and N. Sato, Appl. Phys.
Lett. 83 (2003) 5392.
[5] Y. Kurimoto, T. Shimizu, S. Iizuka. M. Suemitsu,and N. Sato, Thin Solid Film 407 (2002) 7.